Method to test the function of an adaptive echo canceller

ABSTRACT

A method for providing and for testing adaptive echo cancellation in an apparatus including transmission equipment for data communication. The apparatus includes a 2-wire to 4-wire hybrid circuit which receives a signal from a remote end which is cut off for purposes of the testing method. A signal consisting of randomly occurring ones and zeros is applied to the send end of the apparatus. Also included in the apparatus is an adaptive echo canceller which cooperates with the send end and with a filtered signal received from the hybrid circuit to produce a correction signal which is fed back to the adaptive echo canceller and which is also compared with threshold values for purposes of testing the function of the apparatus.

FIELD OF INVENTION

The present invention relates to a method of testing an apparatus foradaptive echo cancellation and more particularly to a method for testingapparatus included in transmission equipment used for digitalcommunication.

BACKGROUND

An apparatus for adaptive echo cancellation of the kind mentioned abovehas been previously described in Swedish patent application 8106444-6.As described in the Swedish application, echo cancellation of, forexample, an incoming analogue signal is effected by correction signalsfrom an adaptive balance filter, the correction signals being added tothe incoming signal and the parameters of the balance filter beingcontinuously or in stepwise fashion updated from a correction unit.

A known method of functionally controlling digital equipment is to loopcircuit data flow from a transmitter to a receiver and to carry out abit error measurement. In this way the transmitter, receiver andfour-wire to two-wire junction of the echo cancellation apparatus can befunctionally controlled.

SUMMARY OF INVENTION

Verification of the function of the echo canceller included in theapparatus involves certain problems, since a measuring signal must besupplied to the two-wire side of the apparatus in such a case. Thismeans that an access relay must be connected to the two-wire circuitupstream of the hybrid circuit for supplying the measuring signal to theecho canceller. Also, the measuring signal must be line coded in the wayemployed in the send units of the apparatus with the object ofsimulating a remote signal. Finally, the echo is changed when the lineis interrupted, which makes it theoretically possible for the echo fromthe measuring equipment to be inhibited, but not the echo from the line.

In the above mentioned apparatus of the prior art, it is of importanceto establish that the correction unit and the digital balance filterfulfil their functions within given error margins. This means that thebalance filter should supply correction values to a summator, which addsthe correction values to incoming signals so that the value of theresidue echo falls below a given limit. If the incoming signal issampled and the correction signal consists of digital values at theaddition, these values should be correct within given intervals.Incorrect values for the correction signals give poorer echo cancellingand can thus give rise to incorrect reception. As examples of causes maybe mentioned: false supply values to the balance filter, deficientsynchronism and fault in the memory cell structure of the balancefilter, e.g. a faulty ROM in the storage unit.

The object with the present invention is to provide a method of testingthe function of an adaptive echo canceller without using an access relayon the line side.

BREIF DESCRIPTION OF DRAWINGS

The invention will now be described in detail with reference to theaccompanying drawing, wherein:

FIG. 1 is a block diagram of an apparatus for adaptive echocancellation, in which the inventive method is utilized, and

FIG. 2 is a diagram of some signals which occur in the apparatusaccording to FIG. 1.

DETAILED DESCRIPTION

An apparatus for adaptive echo cancellation is illustrated in the blockdiagram of FIG. 1, together with a circuit for carrying out the methodprovided in accordance with the invention.

On the send side, the apparatus includes a send unit A for data with acoding unit B connected to the output of the send unit A for recodingsend data to a suitable line code, e.g., a biphase code. The output ofthe coding unit B is connected to a two-wire to four-wire hybrid circuitF, here exemplified as a voltage bridge including the resistors R1, R2and R3 as well as an operational amplifier J. In one of the two-wirepaths a switch SW1 is connected to one terminal of a line transformerTR.

In the reception path of the apparatus, there is a filter circuit (alowpass filter) C with its input connected to the ouput of the amplifierJ for receiving and filtering the incoming line signal from the two-wireline. The output of the filter C is connected to a summator AD andfurther to a receiver detector D. The blocks KE, E, and AD constitute anadaptive echo canceller connected between the send and receive paths andto a summator AD. The filter E conventionally sends a correction signals₁ to the received and filtered signal s₂, a signal s₃ being obtained,which is free from echo, as far as possible. A correction unit KE isconventionally connected between the detector D and the filter E foradjusting the filter parameters in the balance filter included in theecho canceller in dependence on the received and detected signal s₃. Theplus input of a first and the negative input of a second comparator J1and J2 may be connected to the output of the summator AD. The minusinput of one comparator J1 is connected to a first threshold voltage + Tand the plus input of the other comparator J2 is connected to a secondthreshold voltage -T. Each output of the comparators J1 and J2 isconnected to a contact K1 and K2, respectively, included in a samplingcircuit SK. The contacts K1 and K2 are controlled such that sampling ofthe output signal from the associated comparator is done in the same wayas for the received signal s₃ in the block D i.e., synchronous with thesampling rate in block D. An OR-circuit EK is connected to the output ofthe sampling circuit SK.

When testing the echo canceller function according to the inventivemethod, the echo canceller is connected in the normal way and thecontact S1 is closed. The remote end transmitter is shut off so that noline signal occurs over the two-wire circuit 2W, apart from the onegenerated by the local sender (block A). A random bit flow, i.e. a bitflow in which "ones" and "zeros" occur randomly, is applied to the inputdx of the sender A. This bit flow is transferred via the units B, F, andC to the input of the summator AD where it occurs as a signal s₂, andalso to the echo canceller which gives a compensation signal s₁. Forperfect echo cancellation s₁ =-s₂. The signal s₃ on the output of thesummator AD gives the echo signal which has not been cancelled. Thissignal is taken to the inputs of both comparators J1 and J2 and iscompared with the threshold values +T and -T. If this signal s₃ ispositive then the comparison with +T is of interest, while thecomparison with the threshold value -T is of interest if s₃ is negative.After sampling in the circuit SK, there is obtained a "one" pulse or a"zero" (the absence of a pulse) on the output of the OR circuit EKaccording to the following conditions at the sampling instants:

If: s₃ >+T, a "one"(s₃ >0) is obtained

-T<s₃ <+T, a "zero"s₃ ><0) is obtained, and

s₃ <-T, a "one"(s₃ <0) is obtained.

The threshold values +T, -T have a value selected such that the lowestexpected remote signal can be detected even if the residue echo has sucha value of amplitude.

The compared signals are sampled in the same way as a received remotesignal, (e.g., 4 times/bit). An error signal v is obtained from theoutput of the OR circuit EK and consists of a pulse each time the echocanceller E cannot cancel the echo (the signal s₂ in its entirety) to avalue less than /T/. The signal v, which is thus "zero" when thecompensative signal s₃ is less than /T/ and "one" when /s₃ />/T/ can beutilized and evaluated. A quality limit is suitably set for the echocanceller, measured by the quality detector QD in the number of errorpulses ("ones") per second.

It is also conceivable to carry out the measurement of the compensatedsignal s₃ with the contact S1 open or with a nominal load connected tothis contact. Noise disturbances thus disappear from the line, but theecho characteristic is simultaneously changed.

FIG. 2 illustrates both signals s₂ and s₃ in FIG. 1, and the action of agiven compensation from the echo canceller E. The filtered signal s₂ ishere assumed to be essentially sinusoidal and is sampled at the instantst₀, t₁, t₂, t₃, . . . t₆, t₇, t₈, t₉, the echo canceller here sending atthese instants a digital signal level s₁ for the compensation. Forexample, at t₁ there is sent a level -d₁ from the echo canceller to thesummator AD, which lowers the level of s₂ so that the signal s₃ willcome below the threshold +T. This is assumed to take place at theinstant t₃ also. In a similar way, a compensation with +d₂ takes placeat the sampling instants t₆, t₇ so that the level of the signal s₃ comesabove the threshold -T. At the instant t₈ the compensation is assumed tolapse due to possible errors in, for example, one of the stored units inthe echo canceller E, resulting in that the value of s₃ becomes lowerthan -T. Correct compensation takes place once again at the instant t₉.If the error returns a given number of times during a given timeinterval, e.g. during one second, this can be detected and measurestaken. For example, the error may be due to a cell being faulty in thestorage unit. If the storage unit is then addressed by a 7-bit word theerror will occur on an average of every 128th sample.

With the inventive method, the remote signal is thus disconnected and itis a condition that the signal received from the local send side shouldbe lower than a given absolute level /T/ for the echo cancellation to beacceptable.

I claim:
 1. A method of testing the functioning of an apparatus foradaptive echo cancellation included in transmission equipment comprisinga receiver side and a send side to transmit a send signal via afour-wire to two-wire hybrid circuit to a remote end, said apparatusdeveloping an echo cancelling signal (S₁) for received signals (s₂)received from said hybrid circuit, said method comprising the steps ofpreventing signals from the remote end from being provided to theapparatus, and (b) comparing a compensated signal (s₃), obtained fromsaid received signals (s₂) corrected by said cancelling signal (s₁),with at least one threshold (+T,-T), a quality limit of said echocancellation being defined by the number of times the magnitude of saidcompensated signal (s₃) is greater than the absolute value of saidthreshold during a predetermined time interval.
 2. A method according toclaim 1 wherein said compensated signal (s₃) is compared with twothreshold values (+T,-T) of opposite polarity.
 3. A method as claimed inclaim 1 wherein said quality limit is established such that the signalobtained after comparison with said threshold value is sampled and thenumber of signal samples thus obtained which exceed or fall below saidthreshold value (+T,-T) during the predetermined time interval aredetermined.